The field of the invention is esters and processes of making the same from polycarboxylic acids. The present application is particularly concerned with the preparation of dimethyl terephthalate by the esterification of terephthalic acid.
The state of the art of preparing dimethyl terephthalate may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Ed., Vol. 15 (1968), pages 466-467 under the section "Current Commercial Processes for Polymer-Grade Dimethyl Terephthalate", and by reference to U.S. Pat. Nos. 2,876,252 of Rudolf Lotz et al., which issued Mar. 3, 1959 and 3,617,226 of Ferdinand List et al., which issued Nov. 2, 1971, and German Pat. Nos. 1,088,474 (corresponding to British Pat. No. 933,960), of Walter Schimming, published Sept. 8, 1960; 1,188,580 and 1,933,946 of Ferdinand List and Helmut Alfs, published Jan. 21, 1971 (corresponding to U.S. application Ser. No. 39,761, filed May 22, 1970 and now abandoned), the disclosures of which are incorporated herein.
The state of the art of fluidized beds, particularly for use in catalysis and gas-solid reactions, may be ascertained by reference to Kirk-Othmer ibid, Vol. 9 (1966), under the section "Fluidization", pages 398-445.
It is well known that terephthalic acid is a raw material for the production of polyester fibers. Because ordinary commercial terephthalic acid is not pure enough for the polycondensation reaction, the terephthalic acid must be first purified. Because of the unsuitable physical properties of the commercial terephthalic acid, the purified product is very costly. In most cases one uses the dimethylester of terephthalic acid for the purification of the same and this is recovered by alcoholysis with methanol and precondensation with glycol or other diols.
Because of these difficulties, there is substantial interest in a more economical method for the esterification of terephthalic acid. Methanol, because of its low price and molecular weight, is the most suitable alcohol in spite of the chemical and physical difficulties connected with the esterification with methanol. Since terephthalic acid is practically insoluble in boiling methanol, esterification at an adequate rate can only take place in a pressure tight apparatus made of high alloyed steel.
One method has been developed in which terephthalic acid is partially esterified in the gas phase with superheated methanol vapor in the presence of an inert gas as a carrier. This method, as disclosed by U.S. Pat. No. 2,876,252 reacts powdered terephthalic acid with methanol at about 300.degree.C with the addition of a catalytic amount of an acid and it is blown into a reaction pipe. This method results in a relatively low yield with a low purity. According to German Pat. No. (DB-PS) 1,088,474, powdered terephthalic acid is reacted with methanol, and if necessary in the presence of nitrogen, by introduction into the fluidized bed of a catalyst. The mixture is reacted at a high temperature of about 300.degree.C to instantly form a dimethylester which is removed with the excess methanol and reaction water. The dynamic properties of a fluidized bed in general are not particularly advantageous to the reaction since the particle size distribution of the terephthalic acid, the fluidizing of the acid and the catalysts can form undesirable channels in the bed. They must, therefore, be carefully adjusted with respect to one another so that one or both of the components are not carried out of the reactor.
German Pat. No. (DB-PS) 1,188,580 teaches the esterification of terephthalic acid by vaporization in a superheated methanol vapor stream impinging upon a fixed catalyst. The terephthalic acid in pellet form is heated to 315.degree. to 345.degree.C, together with methanol, in a sublimation zone at about 400.degree.C and they are vaporized and esterified in a follow-up reactor at about 300.degree.C. The heat necessary in the sublimation reactor for vaporizing the terephthalic acid can be provided by the heat content of the superheated methanol. This simple esterification method permits heating the terephthalic acid from about 20.degree.C to a medium sublimation temperature of 330.degree.C. Complete evaporation at this temperature requires ten times by weight of methanol heated to 400.degree.C to provide for a continuously operating sublimator.
Compared with this, U.S. Pat. No. 3,617,226 discloses a method and apparatus in which powdered terephthalic acid is in solid contact with a horizontally lying cylindrical container, and reacts with a flowing stream of methanol vapor at a temperature of between 300.degree. and 320.degree.C while being mechanically stirred. The particle size of the catalyst and acid need not be the same and the terephthalic acid dimethylester produced leaves the reactor in the gaseous state. Because the esterification speed at this temperature is extraordinarily high, the reaction of the terephthalic acid and the space/time yield are dependent upon technical factors such as diffusion speed, ratio of the catalyst active surface area to terephthalic acid, material transport and heat exchange and dwell time. With respect to a given unit of time for carrying out the reaction, the dwell time and esterification conversion is directly dependent upon the available catalyst surface and the amount of available heat required for the vaporization of terephthalic acid or dimethyl terephthalate. In this case, the catalyst surface area available per unit of time with respect to the useable volume of the cylindrical container is ascertained, while the transferable amount of heat is limited by this surface area, except for the possible temperature difference between the heating medium and the reaction components. An increase in size of such a cylindrical reactor results in a smaller ratio of surface to volume so that a greater esterification capacity is limited by the technical problems associated with the construction of a large cylindrical reactor with the greatest possible surface area and the necessary stirring mechanism.